1. Field of the Invention
The present invention relates to improvements to heat insulating means for piping subjected to stresses, whether they are thermal, hydrostatic and/or mechanical and the positioning of these insulating means on said piping; it further relates to processes for forming these new insulating means.
2. Description of the Prior Art
In offshore oil fields, the production wells are connected to a production platform or to a subsea collector by underwater ducts disposed on the sea bed. It is usual for a production platform (which provides a first treatment of the crude oil) to be connected to several wells, the number thereof depending on a great number of parameters such as the size and yield of the field, depth of water, etc. . . .
The oil leaves these production wells at a variable temperature (70.degree. to 95.degree. C.) sufficient, in any case, for it to flow into the drilling tube.
When it is a question of light oil, it is then brought to the platform by a conventional metal undersea duct and arrives at the production platform at a temperature, which is variable depending on the length of the pipe, between that of the well head and that of the water in the immediate neighborhood of the duct.
Furthermore, in so far as transporting heavy high viscosity oils or paraffin oils by a metal duct is concerned, the oil conveyed by this duct will be progressively brought to the temperature of the sea water. At a certain distance from the well it will become too viscous or solid deposits will appear causing clogging of the duct. It is obvious that the critical distance for the appearance of this phenomen will be all the smaller the lower the temperature of the sea water and, consequently, the greater the depth at which the duct is located.
The problem also arises for gas pipe lines between the wells and the platform where a considerable lowering of the temperature of the gas causes the formation of hydrates and progressive clogging of the duct.
The need for heat insulating the piping so as to protect it as much as possible from the adverse effects of the low temperatures of the sea environment has therefore become imperative for the operators of "offshore" platforms, so as to reduce as much as possible the temperature gradient of the crude oil between its ouput from the drilling well and its arrival at the production platform, so that it remains low, of the order of 5.degree. to 30.degree. C.
The solution proposed in the prior art for providing insulation of underwater piping of this type, is shown by the arrangement of an outer metal sheath which surrounds the remote crude oil collecting pipe line, the gap between the inner collecting piping and the outer metal sheath being filled with a heat insulating material formed from polyurethane foam. However, this solution has considerable drawbacks both in the economical and in the technological spheres. In fact, whereas the inner collecting piping must withstand the internal pressures developed by the flow of crude oil at a relatively high temperature, the outer sheath must withstand hydrostatic crushing pressures to which the sea environment on the sea bed subjects it, so that it must be made from steel and have a relatively high thickness, adapted for withstanding the high pressures to which it is subjected, so that it is very expensive.
The polyurethane foam injected in situ is relatively unresistant to the high hydrostatic pressures which prevail at the bottom of the sea, so that should the outer metal sheath be accidently perforated, the insulating polyurethane foam is destroyed both by the hydrostatic pressure which it must then withstand directly and without protection and by the sea environment which hydrolyses it; injection takes place without the possibility of checking the quality of the insulation formed, so that it may have uncontrolled and uncontrollable insulating defects.
Furthermore, the outer sheath may comprise solutions of continuity or gaps for welding the successive adjacent tubes on the barge, so as to form the heat insulated collecting pipe line desired; it is then neccessary to leave, in the vicinity of the adjacent junction ends of two successive tubes, a non insulated gap of about 40 cm so as to allow welding; after welding the ends of the tubes to each other, the gap between two sheath sections is made up by positioning a sleeve which is welded to the ends of two adjacent sheath sections, so as to obtain a continuous outer sheath, while leaving however an orifice for the in situ injection of the heat insulating polyurethane foam, which orifice is then closed by a welded plug. So as to prevent any alteration of the insulating material by the temperatures used for welding, during the plug welding operation, it is necessary to protect the insulating material by interpositioning asbestos rings.
The operations for positioning the sheath sections about the inner tubes, welding the adjacent ends of the successive inner tubes together, positioning and welding sleeves in the gaps between the adjacent sheath sections, injecting the insulating polyurethane foam in situ and welding so as to close the injection orifices formed in the welded joins between the sleeves and the sheath sections, with interpositioning of asbestos rings, must furthermore necessarily be carried out on the barge before lowering the piping with its insulating device, to the sea beds on which it is to be laid. The time during which the barge is immobilized on the site is therefore relatively long before the insulated piping effectively connects a drilling well to a production platform, and greatly increases the cost price for laying such heat insulated piping.
It has also been proposed in the prior art to provide piping for transferring the crude oil from the wells to the offshore production platform with an outersheath formed by a tube made from a rigid plastic material such as PVC or polyethylene, the gap between such an outer sheath and the piping being filled with a heat insulating material such as polyurethene foam.
However, this solution is not satisfactory either for the resistance of the outer rigid PVC or polyethylene sheath is insufficient with respect to the hydrostatic crushing pressures which reign in the sea at great depths, more especially greater than 50 meters; in addition, the jointing operations are just as complicated as in the case where the outer sheath is made from steel; furthermore, for laying, repairing and facing up to the consequences of possible damage, problems are met with similar to those which have been set forth above in connection with the heat insulating system formed by polyurethane foam interposed between the piping and the outer rigid plastic material sheath.